The T1/E1 modules contain an onboard Cisco Packet Voice Digital Signal Processor Module 4 (PVDM4) slot, which requires a fourth-generation PVDM4 module. A PVDM4 on the T1/E1 module is necessary for the voice features. The PVDM4 also provides for echo cancellation of up to 128-ms echo-tail length for demanding network conditions.